Gypsum boards having crack resistant edge portions



June 5, 1956 o. E. GIBSON 2,749,263

GYPSUM BOARDS HAVING CRACK RESISTANT EDGE PORTIONS Filed July 21, 1955 Eva/1 its 'flexiblity and strength.

United States Patent 9 GYPSUM BOARDS HAVING CRACK RESISTANT EDGE PORTIONS Oliver E. Gibson, Chicago, Ill., assignor to United States Gypsum Company, Chicago, 11]., a corporation of Illinois Application July 21, 1953, Serial No. 369,443

2 Claims. (Cl. 154-'--45;9)

The present invention relates to an improved type of gypsum plasterboards characterized by having edges 'substantially free of inherent internal cracks or fissures.

More specifically the invention is directed to the formation of a strong edge structure in plasterboards and wallboards so that they will not be subject to longitudinal cracking.

Piasterboard, particularly gypsum wa'llboard, is an article of commerce which has long been on the market. It can be described simply as consisting of a cementitous core encased in fiber sheets. The core is generally set gypsum (calcium sufate dihydrate) which may contain various types of fillers, fibers, aggregates, and voids to lessen its weight, as well as other materials to enhance It is covered on both sides with a fabric sheet which also generally covers both longtudinal sides of the board. These fiber or cover sheets invariably are made of heavy paper or thin cardboard.

Plaster wallboard, particularly of the type which has a straight or square edge, has undergone a series of evolutionary changes, which may be roughly summed up as follows:

At first there were merely two sheets, one on each side of the board, leaving the edge freely exposed. In view of the fact that the core material is inherently of a rather brittle and crumbly nature, it was soon found that such boards would .suifer from loss of core material in handling, and particularly when being nailed into place. Moreover, the joint between two adjacent boards was crude and unsightly and in the early days was usually concealed by means of wooden or metal battens.

Then came the so-called closed edge type of wallboard and plasterboard in which the cover sheets completely surrounded the board both front and back and on the two longitudinal sides, leaving however, the two ends still open.

Many ingeneous devices were used to obtain the proper type of edge in such boards. As the art developed, the standard procedure was to constantly advance a paper cover sheet, the outside of which was intended to form the eventual front of the board, and then to deposit upon this sheet a suitable plastic slurry of calcined gypsum, water and such additives as were required. The width of the core material deposited was less than the width of the paper, which latter was then first folded upwardly to define the sides of the boards, and then was folded back and inwardly so as to come in contact with the core of the back of the board. Substantially at the same time another paper cover sheet, usually the one which was intended to be the back on the finished board, was applied on top of the plastic slurry and which also was of a width substantially equal to that of the final board, being so applied that the back sheet would overlie the inwardly extending portions of the lower or first sheet.

In order to secure the proper adhesion of the two cover sheets to each other, an adhesive is applied to either "ice v the inturned top portions of the lower sheet or 'to the marginal portions of the back sheet overlying the inturned portions, or to both.

It has also become standard practice to apply rollers to the board as it progresses through the board forming machine, so as to press the cover sheets into contact with the core material, and also to assure the proper joint between the overlapping portions of the front sheet and the back sheet.

Attempts were made to have the inwardly extending portions of the front sheet lie on top of the back sheet, but this self-evidently was undesirable, because it would leave that portion where the two sheets overlapped twice as thick, and therefore made it difiicult to produce a board of uniform thickness.

The structure which was finally adopted was to fold the lower sheet over the sides and for a certain distance marginally inwardly over the core material and then to place a back sheet on top of it, this back sheet in some cases even being somewhat tapered or chamferred at the marginal edges so as to secure a smooth joint between the two paper sheets. However, under the influence of the rollers the inwardly extending portions of the front sheet would be depressed downwardly into the core material so that if a cross section of the board were then examined it would be found that the core near the edge would be as much thinner as corresponded to the thickness of the inturned portion of the front cover sheet. In other words there would be an offset or sharp shoulder within the core itself and parallel to the edge of the board.

It was soon found that this shoulder presented many disadvantages, particularly in that it greatly weakened the marginal or side portions of the boards thus made. It appears that this shoulder on the surface of the core has a tendency to produce an incipient cleavage plane which under even comparatively light force or pressure will develop into a crack through the core and which extends all the way along the edge of the board. Though not exactly the same, this ofiset or shoulder may well act somewhat in the same manner as the diamond point scored line used to cut a pane of glass, so that the glass can then snap oif clearly along the scored line.

This possibility of a defect in plasterboards become particularly noticeable during the handling thereof, where many boards were found to be injured, particularly if they had been dropped on their edges during the han-. dling, which has a tendency to start this incipient cracking of the core along the plane of cleavage. Even though the back sheet had been chamfered and would overlie the inturned edges there still would be the indenation of the inwardly extending portions of the lower sheet which was pushed into the plastic mass while it was still plastic, to produce this undesirable ofiset. When such a board therefore was subjected to bending strains near the edge, a line of cleavage would develop, sometimes along the whole marginal portion of the board, so as to make it virtually useless.

This same ditficulty also was very prone to develop when the board was being nailed in place. At times-in particularly aggravated conditions, even the mere act of nailing will start this incipient marginal cracking, with the consequent result that the board may have a slight tendency to move and thereby render any subsequent applicationof a joint treatment ineffective. Obviously any blow it sufficiently strong or inapt will shatter the edge portions of the board and impair its usefulness and- Apcordingly it is one of the objects of the present invention to produce a plaster wallboard having a cemen-' titious core covered on both sides and at the longitudinal edges thereof with fiber or paper cover sheets, applied in such a manner that the cover sheets do not in any way cause a diminution of the thickness of the core, thereby avoiding incipient cleavage planes or points of departure for cracks.

It is a further object of the invention to provide a plaster wallboard in which the inwardly extending portions of the front or face liner sheet where they overlie the back of the core are themselves chamfered to the same extent as the chamfering on the overlying back sheet so that there will be no offset or shoulder in the core.

Other objects of the present invention will become apparent from a further description of the invention when considered in connection with the concurrently filed drawings forming a part of this disclosure in which: Figure l is a cross section through a conventional board such as has hitherto been made and which has both sides as well as the longitudinal edges covered with paper, and which has the aforementioned undesirable internal otfset. In other words this figure represents the prior art from which the present invention is to be distinguished.

Fig. 2 is a cross section of a plasterboard made in accordance with the present invention;

Fig. 3 is a cross section of a plasterboard, the marginal portions of which have been subjected to a strain so as to develop the cracked condition hereinabove discussed;

Fig. 4 is a cross section of a plasterboard made in accordance with the present invention, and which has been subjected to sufficient strain to break the board, but which shows that the crack has developed at some random place far inwardly of the board; and

Fig. 5 is a cross section, on an enlarged plane, of the overlapping sections of paper cover sheets to show the congruent chamfering.

Referring to the drawings, a plasterboard is shown in cross section with the front thereof directed downwardly, which is the position it assumes during manufacture. It is not believed necessary to describe in detail just how such a board is made as this is amply described in Patents Nos. 1,029,328; 1,034,746; 1,330,413; 1,383,- 249; 1,383,254; and 1,540,045, as well as others.

Referring now to Fig. l, which shows the prior art construction, there is the conventional face paper liner 12 which in the process of manufacture is the bottom one but actually in use is the face of the completed board.

This liner 12 in its initial form is wider than the actual width of the core material 13, and not only includes the upwardly extending portions that contact the sides of the core 13 but also the inwardly extending portion 14 which overlies the back of the core 13. This sheet 14 and the overlying cover 15 are joined together by the board formation rollers during manufacture, so that sheet 14 is forced into the plastic core, and therefore makes the marginal portions of the core somewhat thinner than the main portion thereof. The paper cover sheet including the inwardly extending portions is all of the same thickness. The other or back paper liner 15 which covers the core back overlies the inwardly extending portions 14 and is secured thereto by an adhesive 16 (Fig. 5). This will result in a sharp offset 17 that acts as a focus for a cleavage plane that will eventually become a crack if the marginal portions of the board are subjected to bending strain. This condition is illustrated in Fig. 3 where it will be seen that the board is actually cracked, beginning at the fracture point 18, the crack 21 extending clear through to the front sheet 12. The fracture 18 generally occurs near the core break but sometimes may occur in other portions of chamfered edge 19 of the liner 15.

In accordance with the present invention, however, as illustrated in Fig. 2, there is the same front cover sheet 12, the same side portions, but the inwardly extended portions 14 thereof are charnfered from the top so that they produce a taper. The marginal portions of the back cover sheet are also chamfered at 19 to complement the taper so that when the two portions are adhesively secured they will produce a composite or laminated cover sheet which has substantially the same thickness as the back sheet itself. As a result there will be no otiset 17 and hence no inherent line of cleavage.

What happens when a board constructed in accordance with the teachings of the present invention is subjected to marginal strains is illustrated in Fig. 4, where it will be seen that the board has cracked a considerable further distance inwardly from the edge at a point 20.

However, due to the greater strength of the core, a board made in accordance with the invention as herein described is much stronger, and there is but little danger of development of such cracks even under severe handling and careless application.

It could not have been foreseen that chamfering the inwardly extending portions would necessarily solve the problem because it had been known to chamfer the edge or marginal portions 19 of the back sheet in order to obtain a more smooth appearing joint where the two sheets meet, and it was not until an investigation was made of the reasons for the edge cracking that the solution of the problem was found. The present invention is therefore of that type where the discovery of a latent defect is really the mental inventive concept whereafter the solution thereof becomes fairly obvious.

Nevertheless in view of the fact that for more than twenty years plasterboards have meen made with this inherent defect, and no one had found any solution for it, it is evident that applicant has made an invention of far reaching importance in this field.

The invention is of course not limited to the use of a core of any particular material, but as by far most of the boards of course are made with gypsum cores, the invention is claimed primarily in such type of construction. The invention moreover is entirely independent of the use or nonuse of foam or bubble reinforcing agents or other additives which may be encountered in plasterboard cores, and also is of course not limited to straight sided boards as exactly the same problem will be encountered and solved if the edge were rounded, as is sometimes done.

The main point is that there should be no sharp offsets in the core which would serve as incipient points of departure for the development of longitudinal edge cracks.

Accordingly applicant claims:

1. A wallboard comprising a cementitious inflexible core of substantially uniform thickness and having a pair of planar substantially parallel opposite sides, said core being enclosed in fibrous cover sheets, the latter including a first sheet subtending one planar side and two opposite edges of said core, said first sheet having an edge portion thereof folded back into overlying relation with a marginal portion of the second planar side of the core, and a second sheet overlying said second planar side of the core and the folded edge portion of said first sheet, the folded back edge portion of said first sheet and the overlying portion of said second sheet being congruently tapered and adhesively secured to each other; the thickness of said core adjacent the adhesively secured tapered sheet portions being substantially the same as the thickness of the remainder of said core.

2. A wallboard comprising a cementitious inflexible core of substantially uniform thickness and having a pair of planar substantially parallel opposite sides, said core being enclosed in fibrous cover sheets, the latter including a first sheet subtending one planar side and two opposite edges of said core, said first sheet having an edge portion thereof folded back into overlying relation with a marginal portion of the second planar side of the core and resting upon said second planar core side, said second sheet having substantially the same thickness as said first sheet, the folded back edge portion of said first sheet and the' overlying portion of said second sheet being con gruently tapered and adhesively secured to each other whereby the combined thickness of the adhesively secured portions of said sheets is substantially equal to the normal thickness of one of said sheets; the portion of said core adjacent the adhesively secured tapered sheet portions being of substantially the same thickness as the remainder of said core.

References Cited in the file of this patent UNITED STATES PATENTS Thomas et a1 July 12, 1904 Utzman Aug. 6, 1912 Birdsey June 28, 1921 Utzman June 2, 1925 Utzman May 31, 1927 Argy et a1. June 6, 1944 

1. A WALLBOARD COMPRISING A CEMENTITIOUS INFLEXIBLE CORE OF SUBSTANTIALLY UNIFORM THICKNESS AND HAVING A PAIR OF PLANAR SUBSTANTIALLY UNIFORM THICKNESS AND HAVING A PAIR BEING ENCLOSED IN FIBROUS COVER SHEETS, THE LATTER INCLUDING A FIRST SHEET SUBTENDING ONE PLANAR SIDE AND TWO OPPOSITE EDGES OF SAID CORE, SAID FIRST SHEET HAVING AN EDGE PORTION THEREOF FOLDED BACK INTO OVERLYING RELATION WITH A MARGINAL PORTION OF THE SECOND PLANAR SIDE OF THE CORE, AND A SECOND SHEET OVERLYING SAID SECOND PLANAR SIDE OF THE CORE AND THE FOLDED EDGE PORTION OF SAID FIRST SHEET, THE FOLDED BACK EDGE PORTION OF SAID FIRST SHEET AND THE OVERLYING PORTION OF SAID SECOND SHEET BEING CONGRUENTLY TAPERED AND ADHESIVELY SECURED TO EACH OTHER; THE THICKNESS OF SAID CORE ADJACENT THE ADHESIVELY SECURED TAPERED SHEET PORTIONS BEING SUBSTANTIALLY THE SAME AS THE THICKNESS OF THE REMAINDER OF SAID CORE. 